Low-Terahertz Transparent Graphene-Based Absorber

A new, transparent, metal-free absorber, based on the use of multilayer graphene/dielectric laminates (GLs), is proposed for applications in the low-terahertz frequency range. The designed absorber has a total thickness of around 70 µm and consists of a front matching dielectric layer followed by a GL, a dielectric spacer and a back GL. The laminates are periodic structures constituted of graphene sheets separated by 50-nm-thick polyethylene terephthalate (PET) interlayers, while the matching layer and the spacer are one-quarter-wavelength thick and made of PET. The GLs are modeled as homogeneous-equivalent single layers (ESLs) characterized by their sheet resistances Rs. An innovative analytical method is proposed in order to select Rs values optimizing the electromagnetic wave absorption either in low-gigahertz or low-terahertz frequency range. The frequency spectra of the absorption, reflection and transmission coefficients are computed in the range up to 4 THz by using different values of Rs. Then, realistic Rs values of chemically doped graphene monolayers over PET substrates are considered. The designed absorbers are characterized by an absorption coefficient with a peak value of about 0.8 at the first resonant frequency of 1.1 THz, and a 1.4 THz bandwidth centered at 1.5 THz with reflection coefficient below - 10 dB. Moreover, the optical transmittance of the proposed absorbers are computed by means of the optical matrix theory and it is found to be greater than 86% in all the visible ranges

Electrical, mechanical and electromechanical properties of graphene-thermoset polymer composites produced using acetone-DMF solvents

Recently, graphene-polymer composites gained a central role in advanced stress and strain sensing. A fundamental step in the production of epoxy-composites filled with graphene nanoplatelets (GNPs) consists in the exfoliation and dispersion of expanded graphite in a proper solvent, in the mixing of the resulting GNP suspension with the polymer matrix, and in the final removal of the solvent from the composite before curing through evaporation. The effects of traces of residual solvent on polymer curing process are usually overlooked, even if it has been found that even a small amount of residual solvent can affect the mechanical properties of the final composite. In this paper, we show that residual traces of N,N′-Dimethylformamide (DMF) in vinylester epoxy composites can induce relevant variations of the electrical, mechanical and electromechanical properties of the cured GNP-composite. To this purpose, a complete analysis of the morphological and structural characteristics of the composite samples produced using different solvent mixtures (combining acetone and DMF) is performed. Moreover, electrical, mechanical and electromechanical properties of the produced composites are assessed. In particular, the effect on the piezoresistive response of the use of DMF in the solvent mixture is analyzed using an experimental strain dependent percolation law to fit the measured electromechanical data. It is shown that the composites realized using a higher amount of DMF are characterized by a higher electrical conductivity and by a strong reduction of Young’s Modulus

Electromagnetic and electromechanical applications of graphene-based materials

This volume contains the extended abstracts of the contributions presented at the workshop Nanoscale Excitations in Emergent Materials (NEEM 2015) held in Rome from 12 to 14 October 2015, an event organized and supported in the framework of the Bilateral Cooperation Agreement between Italy and India within the project of major relevance "Investigating local structure and magnetism of cobalt nano-structures", funded by the Italian Ministry of Foreign Affairs and the Department of Science and Technology in India

Low-terahertz modeling of graphene/dielectric multilayers using an equivalent single layer in reverberation environment

Low-terahertz field transmission through a graphene/dielectric laminate (GDL) is investigated by modeling each graphene/dielectric bilayer (GDB) as a homogeneous uniaxial anisotropic medium. The effective medium approximation is applied to define the transverse effective complex conductivity of the GDB, which is expressed as a function of the thickness and permittivity of the dielectric layer, and of the graphene sheet conductivity. The graphene/dielectric multilayered structure is modeled as an equivalent single layer (ESL), having the total thickness of the multilayer and characterized by the effective complex conductivity of the GDB. The model is applied to simulate two different sets of chemically doped graphene samples, characterized by measured values of the dc sheet resistance. The transmissivity, reflectivity, and shielding effectiveness of the resulting GDLs are computed in a reverberation environment up to 10 THz by applying the transfer matrix method to both the GDL and to the ESL. The comparison of the obtained results proves the high accuracy of the homogenization-based ESL model

High performance lightweight shield made by thin flexible tunable graphene-polymer laminate

An innovative lightweight and flexible multilayered screen made by electrically doped graphene sheets laminated with thin films of polyethylene terephthalate (PET) is proposed. The shielding effectiveness (SE) of the screen is computed in the low GHz frequency range assuming that the graphene/PET multilayer is modelled as an equivalent single-layer shield having an effective conductivity. A rigorous simulation model is utilized to define the maximum frequency of applicability of the proposed approximate formulation of the SE. A SE of 40 dB is obtained in a wide frequency range, up to more than 200 GHz, with a screen consisting of 24 graphene layers electrically doped and having a total thickness of 0.69 μm

Electromagnetic Field Radiation from MWCNTs and SWCNT bundles: A Comparative Analysis

The electromagnetic field radiated by a multiwall carbon nanotube is predicted in the frequency domain using the equivalent single conductor (ESC) formulation, and it is compared with the field radiated by a single wall carbon nanotube bundle having circular cross section and the same external diameter. The effect of the frequency and of the configuration on the near field level is investigated, in order to predict the risk of electromagnetic interference against nearby components and devices, and to define the most critical conditions

Equivalent Effective p.u.l. Parameters for Reduced Order Circuit of SWCNT Bundle Interconnects

The radio-frequency signal propagation along SWCNT bundle interconnects is analyzed comparing the results obtained applying the exact multiconductor transmission line model and the equivalent single conductor approach. The accuracy and computational effort of different formulations of the effective per-unit-length parameters are assessed and discussed

Optimal terahertz shielding performances of flexible multilayer screens based on chemically doped graphene on polymer substrate

The shielding performances of multilayer screens made of laminated graphene sheets with flexible polymeric interlayer depend on several factors such as thickness of the polymer interlayers, number of laminated graphene sheets, electron transport properties of graphene, and frequency range. Previous studies have highlighted that the frequency dependent graphene conductivity is a function of the charge carrier density, mobility and quantum scattering time, and it is strongly affected by doping and fabrication route. This paper is aimed at the analysis of the shielding performances of laminated graphene/polymer multilayers at terahertz, in order to provide insights on the optimum shield design as a function of frequency. The proposed simulation model accounts for the frequency dispersive properties of the graphene monolayer and of the polyethylene terephthalate (PET), which is considered as flexible polymeric interlayer material. The optimal choice of the substrate thickness is discussed in order to achieve the maximum value of the shielding effectiveness (SE) in the terahertz frequency range. The proposed design procedure is applied to three multilayer shield configurations, which are made of different types of chemically doped graphene. The computed frequency spectra of the shielding effectiveness up to 10 THz highlight the shielding performances of the considered samples

Terahertz Shielding Effectiveness of Graphene-Based Multilayer Screens Controlled by Electric Field Bias in a Reverberating Environment

Innovative multilayered screens made of laminated graphene sheets, with SiO2 as interlayer, over a glass substrate are proposed as electrically-tunable electromagnetic shields at terahertz. The shielding effectiveness (SE) modeling is developed applying the transmission line method in a reverberating environment (i.e., with the screen surface illuminated by an infinite set of plane waves impinging with all possible propagation directions), which is of particular relevance in electromagnetic compatibility (EMC) studies. A new equivalent single layer (ESL) model of the graphene/SiO2 laminate (GL) is also proposed in order to provide a simple computationally-efficient method for the EMC design of GL shielding configuration via numerical EM tools. The approximate model is validated by comparison with the results obtained applying the exact one. The sensitivity analysis of the SE in reverberating environment is performed in the frequency range up to several tens of terahertz with respect to: the electric field bias of the graphene sheets, affecting their chemical potential level through an electrostatic carrier doping; the relaxation time characterizing the electron transport in graphene; the thickness of the SiO2 interlayers. Finally, the SE computed for different shielding configurations against a plane wave with normal or oblique incidence is compared with the one obtained in a reverberating environment in order to highlight the most significant differences at terahertz frequencies

Equivalent single conductor for modeling near field radiated emission of carbon nanotube bundles

Genoa, Ital